The present invention relates generally to the field of heat transfer and in particular to a new and useful apparatus for heating a process fluid using thermosyphons.
It is well known to heat process fluids, such as crude oil, emulsions, amine, etc. using a fire tube heater system. An example of such a system is shown in FIG. 1. The fire tube heater itself is generally a U-shaped tube which extends into a vessel containing the process fluid, and is comprised of three primary sections: a combustion chamber and a burner for forced draft firing or a burner alone for natural draft, the U-shaped tube, and an exhaust stack. The burner, which usually fires natural gas or propane, is used to generate a flame which travels about 1/3 to 1/2 the inlet length of the U-shaped tube. Hot combustion products from the burner continue through the U-shaped tube to the exhaust stack, and into the atmosphere. The hot combustion products release a portion of their heat to the process fluid surrounding the U-shaped tube as they travel through the U-shaped fire tube.
Fire tube heaters have several known drawbacks which require continual maintenance and observation. First, the process fluid surrounding the fire tube is heated unevenly due to the changing heat flux in the fire tube wall as the combustion products release heat. Second, the continued operation of the fire tube results in increased fire tube internal wall temperatures due to scaling on the outer fire tube walls from evaporation and/or cracking of the process fluid. The increased fire tube internal wall temperature causes burn back and increased stresses on the fire tube, which can eventually lead to failure of the fire tube wall and subsequent fire or explosion within the process fluid tank or vessel.
One known alternative to fire tubes operating in natural draft for heating process fluids is found in Canadian Patent No. 1,264,443, System for Separating Oil-Water Emulsion, which has a heat pipe bundle extending between a combustion chamber and a vessel containing an oil-water emulsion. As used therein, the term heat pipe refers to a high performance heat transfer device having the structural elements of: a closed outer container, a capillary wick, and a working fluid exhibiting the desired thermal characteristics. The capillary wick structure returns the liquefied working fluid from a condenser end of the heat pipe back to an evaporator end. The heat pipe uses the phenomena of evaporation, condensation, and surface-tension pumping of a liquid in a capillary wick to transfer latent heat of vaporization continuously from one region to another, without the aid of external work such as gravity, acceleration forces, or pumps. The system of the '443 patent is schematically illustrated in FIG. 2. The vessel 1 receives an oil-water emulsion through an emulsion inlet pipe 2 and which then spreads over a separation plate 3. A substantial quantity of the oil-water emulsion flows down through a downcomer pipe 4 and accumulates in a bottom portion of the vessel 1. A plurality of heat pipes 5 extend at an angle from the horizontal between an external combustion chamber 6 through a wall 7 of the vessel 1 and into the oil-water emulsion 8 which has accumulated in the bottom portion 9 of the vessel 1. Fuel gas for combustion is provided at a fuel gas inlet 10 to the combustion chamber 6 and ignited to heat finned evaporator ends 11 of the heat pipes 5 extending therein. Products of combustion are exhausted to atmosphere via an exhaust stack 12. The finned evaporator ends 11 of the heat pipes 5 are heated in the combustion chamber 6 to cause the working fluid in each heat pipe 5 to travel to their condenser ends 13 which are immersed in the oil-water emulsion 8 in the vessel 1, where heat is released to the oil-water emulsion 8. The heat pipes 5 thus transfer heat into the oil-water emulsion 8 and hasten its separation into free gas which exits via gas discharge pipe 14, treated oil which exits via treated oil outlet 15, and water which exits via water drain 16.
The heat pipe system in Canadian Patent No. 1,264,443 does not disclose particular connections between the heat pipes and vessels nor a burner arrangement in relation to balance heat transfer between the heat pipe evaporator and condenser ends. The heat pipes are also arranged in a single bundle closely positioned adjacent to each other which allows the evaporator ends to operate in high temperature and high velocity combustion gases. Consequently, this requires the condenser ends of the heat pipes to be positioned in high velocity streams of liquid to remove the heat and balance the whole system of heat transfer between the heat source and heat sink.